Materials for steel automotive parts are designed to have higher strengths so as to achieve both crashworthiness and reduction in weight. Independently, the parts upon manufacturing require good workability of material steel sheets. Assume that such high-strength steel sheets, in particular steel sheets having a tensile strength of 980 MPa or more, are subjected to cold working (e.g., cold press forming). Disadvantageously, however, this requires a higher press forming load or causes remarkably low dimensional accuracy.
As a possible solution to the disadvantage, there is a hot press forming technique. The hot press forming is also called hot pressing or hot stamping. In the hot press forming, a material steel sheet is subjected to press forming while being heated so as to perform forming and achieve higher strengths simultaneously. In this technique, the steel sheet at a high temperature (e.g., in the austenite single phase region) is formed or shaped with tools (punch and die), held and cooled at the (forming) bottom dead center to extract heat from the steel sheet to the tools to thereby rapidly cool the steel sheet. Thus, the material steel sheet is quenched. The forming technique, when performed, can give a formed product having good dimensional accuracy and high strengths. The technique can be performed with a lower forming load as compared with cold forming of parts having strengths at a similar same level.
However, the technique requires holding at the bottom dead center for ten and several seconds so as to extract heat into the tools; and during this period, forming of another part is impossible. Unfortunately, one steel member therefore occupies the pressing machine for a long time, and this results in poor productivity.
In addition, the hot press forming requires cooling at a cooling rate of 30° C./second or more. Thus, forming as well as transfer has to be performed within a short time of ten and several seconds; the press forming can be performed approximately only once; and there is a limit to the shape capable of forming by one press forming. Disadvantageously, the technique therefore fails to manufacture parts having complicated shapes. Further disadvantageously, the resulting steel member obtained after the working has high strengths and poor ductility, is not expected to have high shock absorption upon collision, and is limited in uses to which the steel member is applicable.
To solve these disadvantages, investigations have been made to improve the productivity and/or to improve the degree of freedom of forming in hot press forming techniques.
Typically, Patent Literature (PTL) 1 discloses that the productivity could be improved by holding a workpiece at the bottom dead center for a shorter time, separating the workpiece from the tools at a higher temperature, and subjecting the workpiece to a subsequent step. This technique, however, requires rapid cooling (at a rate of 150° C./s in working examples) after forming, thereby requires a special facility design, and is probably poorly versatile, although the technique provides a shorter holding time in the tools. In addition, the manufacturing method specified in PTL 1 performs forming for a short time of ten and several seconds as in conventional techniques, is difficult to perform multistage press forming, and fails to process the workpiece into a complicated shape.
PTL 2 discloses a hot press forming method. In this method, cooling water is injected from tools during press forming so as to shorten the holding time at the bottom dead center and to achieve both high strengths and satisfactory productivity. The method, however, requires complicated production facilities to inject the cooling water from the tools and is not versatile.
PTL 3 to 5 each proposes hot press forming methods. In the method disclosed in PTL 3, a steel sheet heated to 1000° C. or lower is subjected to multistage press forming of two to five stages (steps) in a temperature range of 600° C. or higher and then cooled at a cooling rate of 10° C./second or more. In the multistage press forming, press forming per stage is performed within 3 seconds, and a subsequent press forming is performed within 4 seconds. In the method disclosed in PTL 4, a steel sheet is heated to a temperature range from the Ac3 temperature to the melting point, is partially supported by a punch provided in tools, is subjected to forming at a start temperature higher than a temperature at which all of ferrite, pearlite, bainite, and martensite transformations occur, and is rapidly cooled after the forming. In the method disclosed in PTL 5, press forming is performed so that a formed product is released from press forming tools within 5 seconds after the press forming tools reach the bottom dead center and cooled at a cooling rate of 30° C./s or more so as to allow the formed part to have a hardness HV of 400 or more.
However, further improvements such as adjustment of chemical compositions are probably required so as to surely provide excellent strength-ductility balance.